(a) Field of the Invention
The present invention relates to a system for determining a precise orbit of a satellite and a method thereof. More specifically, the present invention relates to a system for determining a precise orbit of a satellite and a method thereof, in which the system and method are applied to a mission requiring position information that is accurate to within 1 m, such precise position information being obtained by using only L1 carrier phase data and applying a proportional coefficient of the total number of electrons of the ionosphere to an estimation parameter in order to minimize the errors in an ionospheric model.
(b) Description of the Related Art
A global positioning system (GPS) provides three-dimensional information such as the position, the speed, and the direction of a moving object. The GPS is now used for various applications such as in geodetic surveys, general surveying, scientific surveys, and visual synchronization, as well as in moving object navigation systems for the sea, the earth, and the air.
Taken as a whole, the GPS consists of three units: a space unit, a control unit, and a user unit. The transmitter of the GPS satellite generates a C/A (coarse/acquisition) code and a P (precision) code, that are pseudo noise codes, to thus modulate a carrier. The signal modulated in this manner is propagated to a user through an amplifier and an antenna.
The frequency of a carrier for a standard lateral position is 1575.42 MHz (L1 band) and the frequency of a carrier for a lateral position is 1227.6 MHz (L2 band). The L1 band carrier and the L2 band carrier are phase-modulated by the C/A code and the P code. The L1 band frequency uses the P code and the C/A code. The L2 band frequency uses only the P code.
An algorithm for determining a precise orbit of a satellite using the GPS data includes precise orbit dynamics models, measurement models, and an estimation algorithm.
Therefore, according to the algorithm for determining the precise orbit of the satellite using the GPS data, after predicting the orbit for each measurement time by applying precise orbit dynamics models from an initial orbit element, calculated measurement data are obtained by applying measurement models.
An orbit of a satellite is determined through an estimation algorithm using the differences between actual measurement data received from an on-board receiver of a satellite and GPS receivers of ground stations and calculated measurement data obtained by applying the measurement models.
When the on-board GPS receiver uses only received L1 single frequency GPS carrier phase data in a process of determining the precise orbit of the satellite, a path error of a signal through the ionosphere is not completely removed, and accordingly, significant errors exist in the measurement data calculated by the measurement models.
Data errors caused by the path error of a signal must be reduced in order to determine the precise orbit of the satellite.
In the conventional method for determining the precise orbit of a satellite using GPS data, the precise orbit is determined using two different techniques when the on-board GPS receiver generates only the GPS data of the L1 single frequency.
According to the first technique of determining the precise orbit of the satellite using the GPS data, an ionospheric error is removed by combining pseudo range data of a C/A code or a P code with the L1 single frequency GPS carrier phase data.
The GPS satellite broadcasts signals with the C/A code or the P code loaded on the L1 band frequency. The GPS receiver generates the same code, compares the generated code with the code of the received satellite, and measures the time it takes for a signal of the satellite to leave the satellite and to reach the receiver.
Therefore, a distance between the satellite and the receiver is measured by multiplying the speed of light (the speed of the satellite signal) to the time elapsed. The C/A code is formed of a pseudo random noise code that is almost actual noise. Because the obtained distance includes an error, the distance is called a pseudo range.
At this time, because the noise of a code pseudo range is larger than the noise of carrier phase data by 1000 times in the case of the C/A code pseudo distance and by 100 times in the case of the P code pseudo distance, new errors caused by pseudo range noise are added instead of removing the ionospheric error. A degree of precision in the orbit determination deteriorates due to such errors.
In the second technique of determining the precise orbit of the satellite using the GPS data, the total number of electrons of the ionosphere is estimated using the IRI-95 ionospheric model. The path error caused by the ionosphere is calculated using the estimated total number of electrons of the ionosphere.
However, according to the above methods, because only about 60% of the error of an actually existing ionosphere is corrected using only the ionospheric model, the degree of precision in the orbit determination deteriorates.
It is an object of the present invention to provide a system for determining a precise orbit of a satellite and a method thereof, in which the system and method are capable of improving the degree of precision in determining a satellite orbit by determining a precise orbit of a low orbit satellite using L1 carrier satellite data received from a satellite and by removing a path error caused by the ionosphere by applying a proportional coefficient of the total number of electrons of the ionosphere to an estimation parameter.
In order to achieve the above object, there is provided a system for determining a precise orbit of a satellite including a satellite for receiving GPS data from GPS satellites, an international GPS service for geodynamics (IGS) for collecting and processing L1/L2 carrier phase data of reference ground stations of the GPS satellites distributed all over the world, a satellite control system for monitoring and controlling a state of a satellite by receiving telemetry data and transmitting telecommand data through an antenna and for achieving the L1/L2 carrier phase data of the IGS, and an image processing system for processing image data collected by the satellite.
The satellite control system comprises a tracking, telemetry and command (TTC) module for receiving the telemetry data from the satellite, tracking the satellite, and performing a link to the satellite; a satellite operations sub module for extracting the L1 carrier phase data by processing and analyzing the telemetry data received by the TTC module, monitoring the state of the satellite, generating telecommand data to be transmitted to the satellite, and controlling and operating the satellite; and a mission analysis and planning subsystem (MAPS) for determining the precise orbit of the satellite using the L1 carrier phase data extracted by the satellite operations sub module, the L1/L2 carrier phase data of the reference ground stations of the GPS satellites collected by the IGS, and a path error caused by the ionosphere of data, and for analyzing and planning a mission of the satellite.
The MAPS comprises a first data generator for generating double differenced actual measurement data with respect to the L1 carrier phase data extracted by the satellite operations sub module and the L1/L2 carrier phase data of the reference ground stations of the GPS satellites collected by the IGS by pre-processing processor; a second data generator for generating predicted precise orbit data of a satellite by applying precise orbit perturbation models from a priori orbit and attitude elements at the measurement time with respect to the L1 carrier phase data extracted by the satellite operations sub module and the L1/L2 carrier phase data of the reference ground stations of the GPS satellites collected by the IGS; a third data generator for calculating measurement errors through GPS measurement models and generating calculated measurement data with respect to the predicted precise orbit data of the satellite, which is generated by the second data generator; and a fourth data generator for generating a proportional coefficient of the total number of electrons through an operation between the calculated measurement data of the third data generator and actual measurement data of the first data generator, and generating the estimated orbit element and the estimated parameters comprising the proportional coefficient of the total number of electrons of the ionosphere through estimating the parameters and the precise orbit of the satellite.
The precise orbit perturbation models of the second data generator calculate the precise orbit perturbation forces caused by gravitational forces and the perturbation forces caused by non-gravitational forces from a priori orbit and attitude elements at the measurement time, and generate predicted precise orbit data of the satellite with respect to given measurement time according to the equation of motion for a satellite.
The GPS measurement models of the third data generator calculate the measurement models such as ionospheric path delay effect, tropospheric path delay effect, relativistic effect, tide effect of the earth and the ocean, and phase center offset and variation of the GPS receiver antenna.
The third data generator calculates the total number of electrons of the ionosphere using a priori orbit and attitude elements at the measurement time, and the proportional coefficient of the total number of electrons, calculates a delay value of the L1 carrier phase data due to the ionosphere from the total number of electrons of the ionosphere, calculates the proportional coefficient partial derivative of the total number of electrons of the ionosphere using the delay value of the L1 carrier phase data, and calculates a measurement error.
The fourth data generator generates the measurement residuals and the partial derivatives of parameters using the differences between the calculated measurement data of the third data generator and the actual measurement data of the first data generator, and estimates the parameters and the precise orbit of the satellite.
The fourth data generator estimates the state of the satellite and the parameters, which affect the orbit of the satellite, by a weighted least squares batch filter using the measurement residuals and the partial derivatives of the parameters.
There is provided a method for determining a precise orbit of a satellite, the method being applied to a system including a satellite for receiving global positioning system (GPS) data from the GPS satellites, an international GPS service for geodynamics (IGS) for collecting and processing L1/L2 carrier phase data of reference ground stations of the GPS satellites distributed all over the world, a satellite control system for monitoring and controlling a state of a satellite by receiving telemetry data and transmitting telecommand data through an antenna and for achieving the L1/L2 carrier phase data of the IGS, and an image processing system for processing image data collected by the satellite and generating a precise image photograph, the method comprising (a) receiving telemetry data from the satellite and extracting the L1 carrier phase data from the telemetry data and (b) determining the precise orbit of the satellite using the L1 carrier phase data extracted in the step (a), the L1/L2 carrier phase data collected by the IGS, and a path error caused by the ionosphere, and analyzing and planning a mission of a satellite.
The step (b) comprises the step of processing image data collected by the image processing system from the satellite using the precise orbit data of the satellite and generating a precise image.
The step (b) comprises (b1) generating the double differenced actual measurement data with respect to the L1 carrier phase data and the L1/L2 carrier phase data collected by the IGS by pre-processing processor, (b2) generating the predicted precise orbit data of the satellite with respect to the L1 carrier phase data and the L1/L2 carrier phase data collected by the IGS by applying the precise orbit dynamics models from a priori orbit and attitude elements at the measurement time, (b3) calculating a measurement error with respect to the predicted precise orbit data of the satellite, which is generated in the step (b2), through GPS measurement models and generating the calculated measurement data, and (b4) estimating the parameters and the precise orbit of the satellite by performing an operation on the calculated measurement data generated in the step (b3) and the actual measurement data generated in the step (b1), generating the proportional coefficient of the total number of electrons of the ionosphere, and generating the orbit element and the estimated parameters comprising the proportional coefficient of the total number of electrons of the ionosphere.
The step (b4) comprises (i) calculating the measurement residuals and the partial derivates of the parameters to be determined using differences between the calculated measurement data and the actual measurement data, (ii) estimating the parameters and the satellite orbit using the measurement residuals and the partial derivatives of the parameters in the step (i) and determining whether differences between the predicted satellite orbit and the actual data converge within a limited value given by the system, and (iii) storing and outputting the precise orbit elements of the satellite and the estimated parameters when the differences between the satellite orbit estimated in the step (ii) and the actual measurement data converge within the limited value given by the system.
When the differences between the estimated satellite orbit in the step (ii) and the actual measurement data do not converge within the limited value given by the system, the step (b4) further comprises (iv) performing the steps (b2) through (b4) using the estimated satellite orbit element as a priori orbit element at the measurement time and determining whether the differences between the repredicted satellite orbit and the actual measurement data converge within the limited value given by the system.
The GPS measurement models in the step (b3) calculate measurement models such as ionospheric path delay effect, tropospheric path delay effect, relativistic effect, tide effect of the earth and the ocean, and phase center offset and variation of the GPS receiver antenna through modeling.
The step (b3) comprises (i) calculating the total number of electrons of the ionosphere using a priori orbit and attitude elements at the measurement time, and the proportional coefficient of the total number of electrons and applying the GPS measurement models, (ii) calculating the delay value of the L1 carrier phase data caused by the ionosphere from the total number of electrons of the ionosphere, which is calculated in the step (i), and (iii) calculating the proportional coefficient partial derivative of the total number of electrons of the ionosphere and the measurement error using the delay value of the L1 carrier phase data, which is calculated in the step (ii) and calculating the calculated measurement data by applying other GPS measurement models other than ionospheric path delay effect.
In the step (b4), the parameters and the orbit element of the satellite are estimated using the proportional coefficient partial derivative data of the total number of electrons of the ionosphere, which is calculated in the step (iii), and the orbit element at epoch and the proportional coefficient of the total number of electrons of the ionosphere are generated and are applied to determining the precise orbit of the satellite.